In many solid state electric devices, the active volume of the device comprises or lies within a thin sheet, film or layer of crystalline semiconductor material, often in the single crystal or monocrystalline form. This is particularly true of devices or integrated circuits formed from semiconductors such as gallium arsenide, silicon, germanium, indium phosphide, cadmium telluride, etc. Present techniques for fabricating such devices, however, require that the crystalline sheets be formed upon or near the surface of relatively thick substrates of high-purity single crystal semiconductor material, and the use of such substrates for each sheet produced tends to inordinately increase the cost of producing the thin sheets. The substrate costs come about from many causes which include the cost of raw materials, purification, crystal growth, cutting, polishing and cleaning.
It has been recognized that by employing a reuseable substrate all of the above costs could be reduced and many of the costs would be eliminated, and only a minimum of processing cost might be added back. Thus, attempts have been made to employ reuseable substrates in the production of thin sheets of single crystal semiconductor materials, and among these attempts are the following.
Milnes and Feucht have suggested a peeled film technology for fabricating thin films of single crystal silicon. In the suggested procedure, a thin sheet of single crystal silicon is prepared by chemical vapor deposition of a thin silicon film on a silicon substrate previously coated with an epitaxial layer of a silicon-germanium alloy, thus forming a heteroepitaxy structure. The silicon film is then released from the substrate by melting the intermediate layer of silicon-germanium and subsequently peeling the silicon film from its substrate. The substrate may be reused in such peeled film technology. See Milnes, A. G. and Feucht, D. L., "Peeled Film Technology Solar Cells", IEEE Photovoltaic Specialist Conferences, p. 338, 1975.
The Milnes and Feucht peeled film technology was subsequently extended to the production of gallium arsenide solar cells by employing a thin intermediate layer of gallium aluminum arsenide. In this case, the intermediate layer of gallium aluminum arsenide was selectively etched by hydrofluoric acid and the single crystal thin film of gallium arsenide could then be removed from the substrate, which could be reused. See Konagai, M. and Takahashi, K., "Thin Film GaAlAs-GaAs Solar Cells by Peeled Film Technology," Abstract No. 224, J. Electrochem. Soc., Extended Abstracts, Vol. 76-1, May, 1976.
Another technique for using a reuseable substrate to produce thin films of single crystal semiconductor materials is disclosed is U.S. Pat. No. 4,116,751, issued to Zaromb. In this technique, a continuous intermediate layer is also employed between a monocrystalline substrate and an outer material grown epitaxially to the substrate. The continuous intermediate layer can be broken up by cracking, sublimation, selective melting, or other techniques so that the outer layer can be removed from the substrate.
Such prior techniques for reusing single crystal substrates to produce sheets of single crystal material have suffered from certain inherent problems. As an example, these prior art techniques necessitated that the material chosen for the intermediate layer had very special properties. For example, the material employed for the intermediate layer in these techniques was required to be a different material from the substrate material and yet be a material which could be grown epitaxially on the substrate and one which would thereafter allow the sheet to be grown epitaxially on the intermediate layer. This greatly narrowed the class of candidate materials, but beyond these limitations, the intermediate material also had to have melting, sublimation, mechanical, etching or other properties significantly different from those of the substrate and overgrown film. Further, the epitaxial growth procedures required to produce the required heterostructures were found to be difficult to carry out, which further limited the application of such concepts as peeled film technology. Those procedures employing sublimation or melting of the intermediate layer to separate the film from the substrate required elevated temperatures in processing, and such elevated temperatures often had deleterious effects on the device being fabricated.
Techniques employing selective etching were particularly difficult to perform on a practical basis. Since the intermediate layer was relatively thin and continuous, it was found to be difficult to circulate an etchant through the small openings formed at the edges of substrates having films thereon, especially over the large distances required to produce large area sheets. As noted, the preferential etching properties required for the material of the intermediate layer produced further restraints on materials which could be selected for this layer.
As a result, previously suggested approaches to using reuseable substrates were found to be impractical for the production of sheets of crystalline material, particularly large area, thin sheets of semiconductor material, at competitive costs. For any particular semiconductor material, there was an extremely narrow class of materials which could be chosen for the intermediate layer required and the epitaxial growth techniques required to form heterostructures were difficult to carry out. Because of such problems, these techniques never achieved general acceptance for the production of crystalline sheets of semiconductor material.